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Know what to do — and what to avoid — before you have show up for these standard medical tests.
updated 8/23/2009 12:57:56 PM ET 2009-08-23T16:57:56

Medical tests are no fun, so why take them twice? People often end up with inadvertently skewed results, but some simple prep work will ensure that your numbers are accurate.

The test: PSA exam
What it does: Screens for prostate-cancer warnings

What throws it off: Sex. Or specifically, ejaculation. So watch only Cartoon Network the night before.

The fix: Go celibate for a full day before the exam. Ejaculation can raise normal PSA levels for 6 to 24 hours, say researchers at the Washington University in St. Louis school of medicine.

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The test: Colonoscopy
What it does: Checks for colon cancer and other bowel problems

What throws it off: The pre-test laxatives doesn't always work, says Mark Welton, M. D., of the Stanford Cancer Center.

The fix: Help the laxative by walking, which encourages waste to move faster. About 41 percent of patients who walk have a good chance of a clean bowel, compared with 25 percent of those who don't walk, a Korean study shows.

The test: Lipid profile
What it does: Estimates your risk of developing cardiovascular disease

What throws it off: Food and any drink other than water. Some docs also allow coffee during the prescribed 12-hour fast.

The fix: Drink your java black; cream and sugar can alter your triglyceride levels for several hours, which throws off the test. An intense exercise bout can also have that effect. So take it easy at the gym the day before.

The test: Treadmill stress test
What it does: Measures your heart fitness and identifies signs of possible heart disease

What throws it off: Any interference between skin electrode. (That's why your chest hair is promptly Bic'd.)

The fix: Be careful how you clean yourself in the morning. Skip body lotion or powder, both of which can weaken an electrode's read on your heart's electrical signals. (And you'll leave in need of another shower anyway.)

The test: Blood pressure
What it does: Helps monitor your risk of heart disease, stroke, and other ailments

What throws it off: Anxiety over a visit to the doc can spike systolic blood pressure by 5 points and diastolic by 3.5 points.

The fix: Hit the bathroom. Urination reduces blood pressure. Then take slow, deep breaths for 30 seconds; that can drop systolic blood pressure by an average of 6.4 points and diastolic by 2.3 points, say Japanese researchers.

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